Since the discovery that panels of Surlyn® self-heal after being shot with a bullet at subsonic speeds, it has been of interest to understand the underlying mechanism of this self-healing process. Such understanding could lead to the development of new materials that would self-heal in a variety of environments and temperatures. Included would be applications in space structures, which could contain the internal atmospheres should a breach occur due to micrometeoroid impact.
In recent years there have been many research groups exploring several different mechanisms of self-repairing polymeric systems. One such system involves encapsulation of dicyclopentadiene, then dispersing it into a composite matrix containing catalyst. When crack growth breaks the capsules, the monomer is released. The monomer polymerizes when it comes into contact with the catalyst, thereby hindering further crack propagation and regaining some mechanical strength. Another mechanism involves reversible cross-linking via a heat-initiated Diels-Alder reaction. This system offers advantages over traditional hot plate welding techniques, in which chain entanglements or intermolecular interactions are responsible for regaining strength in the material. The Diels-Alder technique forms covalent bonds to repair the damage in the material.
Recent research has revealed that different grades of Surlyn®, a polymer manufactured by DuPont, underwent recovery after being shot with a 9 mm bullet at subsonic speeds. Although the remaining impressions varied in appearance after impact, depending upon bullet impact speeds, no hole remains in the sample in all cases when shot at ambient temperature. This behaviour is the reason this polymer is used in targets at firing ranges under the name React-A-Seal®.
Surlyn® is a copolymer of ethylene-methacrylic acid, which is neutralized with different counterions. The properties of these and similar ionomers have been studied since the late 1960s. Although these polymers possess many qualities distinctive to ionomers, such as ionic cluster formation, no conclusions have been determined as to why these polymers recover after bullet impact. It has been speculated that ionic cluster formation may be a driving force for the material to flow back together after bullet impact. No subsequent studies on other ionomeric systems were initiated to support this claim, and no satisfactory explanations have been provided to explain the self-healing characteristics of these polymers.
There thus remains a need for determining the mechanisms by which such polymers self-heal, the development of additional self-healing polymers and the production of structures such as fuel tanks fabricated from polymeric materials that exhibit such self-healing characteristics.